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Quantum Electronics and Laser Science Conference

Influence on parametric four-wave mixing of a two-photon interference effect

W. R. Garrett, Richard Wunderlich, Roger C. Hart, Mary Anne Moore, M. G. Payne

Published Apr 24, 1989• 0 citations
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Abstract

If an extended Na vapor column (heat pipe configuration) is subjected to strong laser pumping near two-photon resonance with 3D3/2,5/2 or 4D3/2,5/2 states at pressures above tens of millitorr, strong stimulated emissions are produced by amplified spontaneous emission (ASE) or stimulated electron in hyper-Raman scattering (SEHRS) and by phase matched parametric four-wave mixing (PFWM). These new electromagnetic fields that are generated within the near-resonant nonlinear medium can strongly influence the total atomic response and produce several saturation and total cancellation types of effect.

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Experimental investigations of two-photon enhanced stimulated hyper-Raman and parametric four-wave-mixing emissions are described for tuning unfocused laser beams near two-photon resonance with 3D and 4D states in sodium vapor. Over wide ranges of vapor pressures, path lengths, and laser power densities, the gains for stimulated hyper-Raman (SHR) scattering and for parametric four-wave mixing (PFWM) can become very greatly reduced from values expected from simple considerations. The suppression of gain can arise from interferences produced at two-photon and three-photon resonances, and from ac Stark shifts produced by internally generated near-resonant fields. Quantitative comparisons are made between observed and theoretically predicted values for SHR and PFWM production. Of 12 predicted consequences of the interferences and Stark shifting on the nonlinear behavior, ten are demonstrated through the experiments.

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Experimental studies have been made of conjugate wave generation in a Na metal vapor heat pipe. Detailed measurements of gain profiles, pump beam absorption, and parametric wave generation at infrared frequencies for phase conjugation near the two-photon resonance with the Na 4d2D5/2 state, yield details of the effects of parasitic processes on degenerate four-wave mixing. Studies are conducted with an excimer-pumped dye laser system with ~4 ns pulses of < 10mJ/pulse, traversing Na vapor heat pipe. Included in the measurements are pump beam depletion, probe and conjugate beam intensities, and infrared light which is generated by the strong pump beams through stimulated Raman (SR) and amplified spontaneous emission processes. The role of ac Stark shifting in degenerate four-wave mixing near the 4d level is studied. We find a new effect involving a large contribution to the ac Stark effect from the internally generated SR photons. We observe a rapid onset with pressure of stimulated Raman emission accompanied by a dramatic increase in the ac Stark profile of the 4d level, and the sharp onset of gain saturation for the phase conjugate signal, due to the competition of these two processes. This effect results in an ac Stark coefficient which can be very nonlinear in laser power and Na pressure.

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DESTRUCTIVE INTERFERENCE BETWEEN PARAMETRIC FOUR-WAVE MIXING AND THREE-PHOTON SCATTERING

Yaakov Shevy, M. Rosenbluh

The propagation of a nearly resonant strong laser in Na vapor results in a number of non-linear optical processes such as parametric four-wave mixing (FWM), three-photon scattering (TPS), and Raman scattering.1 The Raman and TPS processes involve the emission of radiation at specific and unique frequencies determined by the detuning of the laser from the atomic resonance, δ, and the Rabi frequency, Ω. Parametric FWM, however, need only satisfy the condition that 2ωL = ω3+ ω4, and can occur over a broad range of frequencies. The frequency range is restricted only by the available gain for the FWM process. The gain depends strongly on the possibility of phase matching at the given frequencies involved in the interaction, as well as on nearly all of the experimental parameters such as relaxation rates (collisional and otherwise), Rabi frequencies, laser detuning, the length of the Na interaction region and the Na vapor pressure.

•Jul 18, 1988•0 citations

Suppression and shifting of odd-photon resonant excitations and stimulated hyper-Raman emissions

W. R. Garrett, M. G. Payne

When a principal atomic transition (dipole allowed from the ground state) is driven by odd- multi-photon excitation or by stimulated hyper-Raman scattering, a nonlinear multi-wave mixing field is generated at the transition frequency. A number of dramatic effects on such odd-photon mediated processes can ensue as a result of the internally generated wave-mixing fields. Excellent agreement between theory and experimental studies in Xe and in metal vapors is illustrated for a number of predicted effects including: suppression and strong shifting of resonance lines and hyper-Raman emissions under single and multi-laser excitation. We illustrate the dependence of the interference-related effects on pressure, oscillator strength, wavelength combination and relative propagation directions of pump laser beams. Contrary to most other nonlinear effects, we show that none of the features (in the semi-classical regime) depends on the intensity of the driving laser field.

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Apr 1, 1992•0 citations
If an extended Na vapor column (heat pipe configuration) is subjected to strong laser pumping near two-photon resonance with 3D3/2,5/2 or 4D3/2,5/2 states at pressures above tens of millitorr, strong stimulated emissions are produced by amplified spontaneous emission (ASE) or stimulated electron in hyper-Raman scattering (SEHRS) and by phase matched parametric four-wave mixing (PFWM). These new electromagnetic fields that are generated within the near-resonant nonlinear medium can strongly influence the total atomic response and produce several saturation and total cancellation types of effect.
Experimental investigations of two-photon enhanced stimulated hyper-Raman and parametric four-wave-mixing emissions are described for tuning unfocused laser beams near two-photon resonance with 3D and 4D states in sodium vapor. Over wide ranges of vapor pressures, path lengths, and laser power densities, the gains for stimulated hyper-Raman (SHR) scattering and for parametric four-wave mixing (PFWM) can become very greatly reduced from values expected from simple considerations. The suppression of gain can arise from interferences produced at two-photon and three-photon resonances, and from ac Stark shifts produced by internally generated near-resonant fields. Quantitative comparisons are made between observed and theoretically predicted values for SHR and PFWM production. Of 12 predicted consequences of the interferences and Stark shifting on the nonlinear behavior, ten are demonstrated through the experiments.
The nonlinear response of sodium vapor with a laser tuned near the 4D two-photon resonance is investigated over a large range of sodium concentrations and pump intensities with emphasis on the influence of coherent cancellation effects on the induced polarizations and the primary mechanism for the generation of the new frequencies. Over a wide range of circumstances, the response of the medium is dominated by parametric four-wave mixing as compared to amplified spontaneous emission or stimulated electronic hyper-Raman scattering. Secondary four-wave-mixing fields are generated by stimulated Raman scattering of one parametric four-wave-mixing field and a coupling of these fields with the driving laser field. A sharp onset of saturation of the parametric four-wave-mixing signals with sodium number density and a transition from a quadratic to a linear power dependence is observed. Both effects are explained in terms of the two-photon cancellation effect.
Experimental studies have been made of conjugate wave generation in a Na metal vapor heat pipe. Detailed measurements of gain profiles, pump beam absorption, and parametric wave generation at infrared frequencies for phase conjugation near the two-photon resonance with the Na 4d2D5/2 state, yield details of the effects of parasitic processes on degenerate four-wave mixing. Studies are conducted with an excimer-pumped dye laser system with ~4 ns pulses of < 10mJ/pulse, traversing Na vapor heat pipe. Included in the measurements are pump beam depletion, probe and conjugate beam intensities, and infrared light which is generated by the strong pump beams through stimulated Raman (SR) and amplified spontaneous emission processes. The role of ac Stark shifting in degenerate four-wave mixing near the 4d level is studied. We find a new effect involving a large contribution to the ac Stark effect from the internally generated SR photons. We observe a rapid onset with pressure of stimulated Raman emission accompanied by a dramatic increase in the ac Stark profile of the 4d level, and the sharp onset of gain saturation for the phase conjugate signal, due to the competition of these two processes. This effect results in an ac Stark coefficient which can be very nonlinear in laser power and Na pressure.
The propagation of a nearly resonant strong laser in Na vapor results in a number of non-linear optical processes such as parametric four-wave mixing (FWM), three-photon scattering (TPS), and Raman scattering.1 The Raman and TPS processes involve the emission of radiation at specific and unique frequencies determined by the detuning of the laser from the atomic resonance, δ, and the Rabi frequency, Ω. Parametric FWM, however, need only satisfy the condition that 2ωL = ω3+ ω4, and can occur over a broad range of frequencies. The frequency range is restricted only by the available gain for the FWM process. The gain depends strongly on the possibility of phase matching at the given frequencies involved in the interaction, as well as on nearly all of the experimental parameters such as relaxation rates (collisional and otherwise), Rabi frequencies, laser detuning, the length of the Na interaction region and the Na vapor pressure.
When a principal atomic transition (dipole allowed from the ground state) is driven by odd- multi-photon excitation or by stimulated hyper-Raman scattering, a nonlinear multi-wave mixing field is generated at the transition frequency. A number of dramatic effects on such odd-photon mediated processes can ensue as a result of the internally generated wave-mixing fields. Excellent agreement between theory and experimental studies in Xe and in metal vapors is illustrated for a number of predicted effects including: suppression and strong shifting of resonance lines and hyper-Raman emissions under single and multi-laser excitation. We illustrate the dependence of the interference-related effects on pressure, oscillator strength, wavelength combination and relative propagation directions of pump laser beams. Contrary to most other nonlinear effects, we show that none of the features (in the semi-classical regime) depends on the intensity of the driving laser field.